Decking Installation

Before undertaking any construction work you must consult with your local authority to check that any construction requirements are met and to determine if a Building consent is required. A qualified tradesman should be consulted where expert services may be required. 

Click here, to find the Decking Installation Manual download.


  • On-site Storage & Handling: Care must be taken to ensure that timber and accessories are kept clean and dry, and are not damaged whilst in storage awaiting application. Extra care is to be taken also while handling timber profiles to ensure that they are not marked or damaged. Upon delivery, timber is to be stacked flat level on bearers/dunnage that are a maximum of 900 mm apart and atleast 100 mm off ground. Timber should be either stored inside an enclosed building covered with an additional waterproof layer and protected from the elements when stored inside.
  • Ground Clearance: The sub-framing must be constructed so that decking boards are a minimum height of 300mm above finished ground level. Adequate ventilation is critical within sub-frame to prevent moisture build-up under the deck surface. The boards must be free draining, and must dry out after periods of intermittent wetting. It is also necessary to ensure the area below the deck is free draining to prevent water pooling below deck surface. This is especially critical when installing a deck over a waterproof membrane. 
  • Joint Spacing: Joists should be at maximum 450mm between centres but can be extended (up to 600mm) by using larger cross sections (100 x 32mm, 140 x 40mm, etc), an engineer should be consulted for specifications and joinery. Refer to decking installation manual for more info on spacing


  • Docking out Defects: Before coating or installing boards, check for any defects that may require docking out. Important: All docked ends of coated decking profiles will require coating with a minimum of one coat of primer or stain.
  • Length Sorting: Sort the timber lengths to optimize usage.
  • Coating: Although not very common in New Zealand, in some cases coating your deck will extend its life. Where required, we recommend a minimum of one coat of a suitable exterior timber decking coating or oil is applied to all four sides plus cut ends of the decking prior to installation.

Fastenings & Fixings

  • Depending on the species selected, deck fixings may be either galvanized iron or stainless steel. Check the Species recommendation to ensure compatibility
  • Fixing placement for a 90mm wide board to be 12.5mm in from each edge of profile, and for 135mm or wider board, 15mm in from each edge of profile. 
  • Align profiles perpendicular to joists. Pre-drill all fixing holes, using counter sinking drill bit that is slightly smaller than the screw shank, to reduce the risk of splitting but ensure a snug fit. Pilot holes at the end of boards should be the same diameter as the screw shank. 
  • ITI Timspec recommends using stainless steel, countersunk twin thread decking screws for all decking profiles. Fixings must be long enough to achieve a minimum of 30mm framing penetration.
  • Pilot Holes: Pre-drill pilot holes slightly smaller than the screw gauge before fixing.
  • Stagger Fixings: Staggering the fastenings across the joists will help reduce the risk of splitting.
  • Expansion Gaps: An expansion gap is required between the boards to allow for movement and drainage of surface water. The gap required depends on the species and profile size. See table below for correct spacing.


  • Placement: Joins must be centered over a supporting joist.
  • Method: Decking should be cut square and butted together. Planing an angle 2-3mm chamfer off the end of each board will mask any unevenness in height. 
  • Fixing Location: Fixings should be 20mm in from the butted join and angled toward the centre of the joist to maximize holding performance.
  • For joining boards end to end, use a butt joint centered over a supporting joist, ensuring there are fixings in board on either side of join. 
  • Fixings may need to be on a slight angle to ensure correct penetration of the joist and ensure fixings are not too close to the end of the profile. 


 Find our Cleaning and Maintenance Guide.

  • For most areas (especially coastal, industrial, and inner-city areas) maintenance should be performed at least once every 12 months. Maintenance may be required as often as once every six months, depending on the level of dirt, organic matter or airborne pollutants in the environment. For sheltered, inland locations, maintenance should be performed once every 12 to 18 months. 
  • Wash all exterior surfaces with low pressure water to remove dust, dirt and other contaminants; a). Extra attention should be given to areas that are not exposed to rain as sheltered areas below eaves where contaminants may not be washed away; b). Do not direct high pressure water (water blasters) on any part of the hardwood timber; c). Do not use harsh or abrasive chemicals on deck surface.
  • Use a soft bristle nylon brush or broom with water and an appropriate cleaning agent (refer to paint manufacturer for correct cleaning agent specific to their coating system) to remove stubborn or persistent dirt and contaminants.
  • Inspect all surfaces and junctions for signs of damage, wear-and-tear, or coating breakdown. Where coating surface has broken or remedial action is required: a). Remove all damaged or loose coating (may involve sanding back to solid timber); b). Spot prime any bare timber (If repairing paint); c). Re-coat with minimum two coats of the product originally used to initially overcoat the timber.
  • Repairing or replacing any damaged or deteriorated items: a). Small isolated areas of dry rot in timber can be cut out and filled then primed and coated; b). For larger areas of deterioration: remove and replace either damaged section or the entire board. Prime and coat the replacements as required;
  • Where a coating is applied, periodic re-coating is required to ensure the integrity of the coating is sustained. This will generally mean applying another exterior coat every 4 to 10 years (dependent on the coating used and building location) after washing and maintenance. 

Fastener Types

Fastener Types
Nails We do not recommend using nails for hardwood decking profiles.

Screws These are suitable for decking boards up to a maximum size of 140x19 mm, anything larger may require bolts.

Hidden fasteners These are relatively new to the marketplace and different brands work slightly different to each other but all are essentially fixed to the joists and attach to the side of the decking board.  The end result is a very contemporary clean-looking deck because no nail or screw heads are visible on the surface.

Expansion Gaps

Expansion Gaps are essential for many reasons, but primarily to provide ventilation and drainage and to allow for the timbers natural tendency to shrink and swell. This will be dependent on the amount of moisture in the immediate environment. Three factors need to be considered in determining the gap between boards:

  • Moisture Content: The moisture content of the framing must not exceed 20% at the time of fixing the decking as problems may occur later and cause excessive timber movement if framing is too wet. 
  • Species: Expansion gap for each species is different. Refer the following table for information. Decking species at ITI Timspec include Kwila, Vitex, Spotted Gum and Accoya®
  • Size (width of the board): Wider boards will shrink/swell more than narrower boards so require a larger expansion gap.

Recommended Expansion Gaps for Hardwood Decking

This table gives recommended expansion gaps for Timber Decking:

  90-120 mm Wide 130-140 mm Wide
Species Green / Shipping Dry Air Dried / Kiln Dried Green / Shipping Dry Air Dried / Kiln Dried
N/A 3-4mm N/A 4-6mm
Blackbutt 3mm 4mm Not recommended 6-8mm
Jarrah Not recommended 5mm Not recommended Not recommended
Kwila 3mm 4mm Not recommended 6-8mm
Saligna Not recommended 5mm Not recommended Not recommended
Vitex 3mm 4mm Not recommended 6-8mm

For more information download our hardwood decking brochure, visit our downloads page.